1. Technical Field
The present invention relates to the field of fabricating optical devices and, more particularly, to a method for fabricating optical devices, including, for example, couplers and crossovers, by individually assembling multiple wafers, each containing at least one planar optical waveguide (POW), and bonding the wafers together to form said optical devices and the optical devices fabricated by such a method of fabricating.
2. Description of the Related Arts
The defining element of planar optical waveguide (POW) technology is the optical waveguide itself, a region of relatively high refractive index which is typically much longer than it is wide or deep, formed, for example, by diffusion on a substrate wafer of lower refractive index material. Light is bound to the high-index guide, eliminating the natural diffraction spreading of a free light beam and allowing the light to be turned through curving paths. Typically, the waveguides are fabricated near the (approximately planar) surface of the substrate wafer, like the interconnections in an electronic integrated circuit.
Current planar waveguide technology has been focused on a single layer of waveguides, which, for example, severely limits crossing of waveguides. A few research results have explored three dimensional couplers in semiconductor materials, but that work requires multiple epitaxial growth processes of layers on a single wafer, an expensive process which has not been extended to glass waveguides. Presently, as many as seven layers may be grown on a substrate with different properties, but with each additional layer, complications in tolerances result. Recently, Raburn et al. in their paper Double-Bonded InPlInGaAsP Vertical Coupler 1:8 Beam Splitter, published in “IEEE Photonics Technology Letters”, Vol. 12, December, 2000, disclose a three layer double bonding process where the three layers are grown using vapor deposition. A first wafer with etched waveguides is bonded to a blank second wafer. The second wafer is then etched and a third wafer bonded. A waveguide of one layer may be at a different height than another waveguide of another layer.
Direct wafer bonding processes are known, for example, from Zhu et al., Wafer Bonding Technology and its Application in Optoelectronic Devices and Materials, published in “IEEE Journal of Selected Topics in Quantum Electronics,” vol. 3, pp. 927–936, 1997 and from Black et al., Wafer Fusion: Materials Issues and Device Results, published in IEEE Journal of Selected Topics in Quantum Electronics,” vol. 3, pp. 943–951, 1997.
Consequently, there remains a need in the art for improved methods of fabricating optical devices and, in particular, couplers and cross-overs having different properties than conventional optical devices.